MailPluginFix is a free tool that will help you to fix your incompatible Mail.app plugins after a macOS update.
Just start the application and you will see a list of all incompatible plugins for your current Mail.app installation. Just select the ones you would like to fix and press the start button in the toolbar. All selected plugins will get patched with the UIDs of the current Mail.app version and also all disabled plugins will be enabled again.
If you're using Mojave, it might be additionally necessary to re-enable the plugins in the Mail.app (Preferences / General / Manage Plug-Ins).
If this tool was helpful for you we would appreciate your donation to support our coffee consume: